I suppose by this question what you're asking is "what is the mechanism of emission of thermal radiation by a hot object?"
The answer to this is that there is, effectively, no one mechanism, because thermal motion is, in a sense, the "most random" motion that a physical system can possibly have. From a classical mechanical point of view, charged particles jostling around randomly will emit a random field of radiation as they produce small fluctuations in the field nearby to them, which then must, by Maxwell's equations, propagate outward as waves. At the surface, these waves are able to escape the object. If you want to think about this, think about dipping your fingers in water and then fluttering them around. There is no one type of motion that produces this radiation - rather, it is due to all of their non-linear, accelerated motions together which are disturbing the electromagnetic field.
Of course, the prediction one derives if one takes this seriously is that the power actually ends up being effectively infinite - the famous "ultraviolet catastrophe" - and that's obviously garbage, so we need to talk about quantum mechanics. In the case of quantum mechanics, particles are more structured with regard to how their energies can change and this is what obviates the difficulty, but nonetheless again all possible transitions, which can result in the emission of a photon, are fair game and all of them will contribute radiation, just as all movements which in classical mechanics would result in emission are fair game in the classical scenario. This can include shell transitions, but also can include, and especially in metals, transitions in band levels as well.
In the most general sense, thermal emissions result from excitation of every single possible way that energy is capable of escaping from the system at a microscopic level - no, in fact, every possible way, period: in theory, they could even excite macroscopic escapes such as ejecting a portion of the material in a spontaneous self-fracturing, but the probabilities of these are incalculably small and thus, effectively, "suppressed". (The microscopic version of this - ejection of single atoms - does occur with relative frequency, and this results in evaporation.) This also even is not only how radiation is possible (thermal excitation of radiative escape paths) but also conduction, as well: such can be understood as thermal excitation of modes of escape where energy escapes by being kinetically transferred to neighboring matter.